1. Field of the Invention
The present invention relates generally to the field of material handling equipment, and more particularly but not by way of limitation, to an improved downstacker assembly for stacking paperboard products at high speeds and for forming multiple-tier loads of stacks.
2. Discussion of Prior Art
The packaging of products in paperboard containers or boxes has increased so much over the years that a very large packing industry has emerged. It is common to cut paperboard container blanks from planar sheets of corrugated composition via rotating dies that operate at very great linear speeds. The blanks are then removed of excess trim and stacked flat in bundles for shipment to points of usage.
The work function which is addressed by the present invention is that of receiving, detrimming and stacking paperboard flats or blanks from the aforementioned rotary die cutters. The linear exit speeds of these paperboard blanks can approach hundreds of feet per minute, with up to one thousand feet per minute and more being possible if the rotary die capability speeds are matched. Unfortunately, no prior art stackers have been capable of reaching and maintaining such capability.
Interestingly, the stacking function has long been addressed, such as by the continuous layboy taught by Lamb in U.S. Pat. No. 2,205,767 (issued June 25, 1940), who even then recognized that much had been accomplished in the mechanization of stacking. In essence, Lamb presented a main receiving table which was lowered at the rate of stack build up, and a finger table which was capable of moving into position to temporarily catch the falling blanks while unloading the main table.
Ward and West, in a more recent and perhaps more complete teaching in U.S. Pat. No. 4,500,243 (issued Feb. 19, 1985), taught a blank stacking apparatus utilizing the feature of receiving the paperboard blanks onto an inclined vacuum conveyor to deliver same to the lower run of an overhead vacuum conveyor disposed over a dropping chute. Release of the blanks is achieved by timing the interruption of vacuum suction to the belts (by the supporting pulleys) just over the dropping area. As the falling blanks settle upon an underlying conveyor, the conveyor is withdrawn downwardly as the stack builds. Side spanker assemblies tamp the stack to align it. Once a stack is completed, a set of tines is extended to catch the blanks during unloading of the stack. The disadvantage of the Ward and West downstacker is the difficulty in maintaining the timing sequence required thereby at high operating speeds.
None of the prior art stackers known to the present inventors achieves continuous, high speed stacking of paperboard blanks and the like. It is to that end to which the present invention is directed.